Among the greatest public health concerns today is the rapid rise in obesity in the pediatric population, which now exceeds 18% in children (6-11 y) and 17% in adolescents (12-19 y), while obesity among young adults (20-39 y) approaches 30%. Though much has been learned regarding the regulation of body weight and the development of adiposity, the prevalence of obesity continues to rise, demonstrating that even today the factors that predict healthy weight maintenance remain poorly understood. In fact, the transition from overweight adolescent to obese adult is almost inevitable, with the probability of obesity in adulthood exceeding 50% among children >13 years of age whose BMI percentiles meet or exceed the 95th percentile for age and gender. But what if we could deter the expected transition from overweight adolescent to obese adult and intervene on a young person at a critical time when he/she is beginning to feel empowered by independence? This is the essence of the Training Interventions and Genetics of Exercise Response (TIGER) Study. The TIGER Study began in 2003 with the goal of identifying genetic factors that influence physiologic response to exercise training, while introducing sedentary college age individuals to regular exercise. Subjects in the TIGER Study currently underwent 30 weeks (2 semesters) of exercise training, 3 days per week for 40 minutes at 65-85% of age-predicted maximum heart rate. More than 80% of subjects lost weight or maintained their weight within 2 kg, and only 18% of subjects gained more than 2 kg. We have identified preliminary associations between gene variation and exercise dropout in the first phase of the study, and in phase 2 of the TIGER study, we will build on these observations. We will focus on three primary areas of investigation: 1) formal evaluation of the TIGER Study intervention protocol to achieve long-term change in the participants; 2) investigation of gene expression patterns in skeletal muscle and subcutaneous adipose tissue as a strategy for identifying genes related to exercise response; and 3) investigation of the association between genetic variation and exercise adherence. Subjects will undergo 35 weeks of exercise training based on the established TIGER protocol, and subjects will be re-contacted 6, 12, and 24 months following completion of the study protocol and questioned regarding current exercise habits and body weight. In addition, genetic analysis of candidate genes related to exercise behavior and neural signaling will be undertaken in order to identify genetic factors that may influence exercise adherence. Finally, patterns of gene expression in subcutaneous adipose, skeletal muscle, and peripheral leukocytes will be analyzed at baseline and following exercise training in order to identify factors that influence physiologic change consequent to exercise training. Results from this study are designed to enlighten our current understanding of how and why individuals respond and/or persist in exercise, with the ultimate goal of formulating better and more efficacious interventions for obesity.